1. Field of the Invention
The present invention relates to a fuel cell system and a control method therefor. More specifically, the present invention relates to a portable-type direct-methanol fuel cell system (DMFC: Direct Methanol Fuel Cell) and a control method therefor.
2. Description of the Related Art
When starting a direct methanol fuel cell system, which uses a methanol aqueous solution as a fuel without depending upon a reformer, under a normal ambient temperature, it is desirable that the fuel cell stack is brought to a predetermined temperature (approximately 65° C. at which an appropriate amount of power can be generated within a short period of time. As a proposed solution described in OHM General Engineering Magazine, July 2003 issue, Contents, Volume 90, issue 7: Technical Trend in Portable-Type Fuel Cell Stack; by Eichi Nomura, Yuasa Corporation, hereinafter referred to as Non-Patent Document 1, for example, discloses a technique of increasing the temperature by supplying the fuel cell stack with a thick methanol aqueous solution at the startup in order to promote a crossover in the fuel cell stack. Heat generated by the crossover and heat generated by the power generating reaction combine to accelerate the temperature increase. When the fuel cell stack is supplied with the thick methanol aqueous solution, methanol crossover increases, and the heat generating reaction on the cathode side increases, thereby enabling a reduction in the amount of time that is necessary to attain the desired temperature.
However, Non-Patent Document 1 does not mention concentration control of the methanol aqueous solution. If startup and shutdown of the system are repeated in a short period of time, the methanol aqueous solution becomes too thick because methanol fuel is inputted every time the system is started. The excessively thick solution causes excessive reaction and rapid temperature increase in the fuel cell stack, leading to temperature overshooting in the fuel cell stack and making the temperature control very difficult. Another disadvantage is reduced methanol fuel utilization efficiency due to too much input of the methanol fuel.